As oceans warm, coral reef fish might prefer to move rather than adapt

In experiments using a fish found in coral reefs around the world, the blue-green damselfish, Chromis viridis, researchers found that the fish were capable of adapting to living in water 2-4 degrees Celsius above their normal summer temperatures; however, when given the slightest chance, the fish moved to cooler water.

"When fish have to adapt to increased temperature, there are physical consequences. They may not be able to handles stress, or reproduce, or even grow," said marine scientist Dr. Jacob Johansen from The University of Texas at Austin. "But, when they seek out temperatures that they've evolved to be in over thousands of years, they can mitigate the impact of increasing temperatures and not sacrifice critical physiological processes."

Johansen and fisheries biologists from the University of Copenhagen and James Cook University collaborated on the study, published in the journal Global Change Biology.

Marine fish are faced with a tough decision. The ocean is warming at a faster rate than occurs on land, because oceans operate as sinks -- trapping in the heat. Many ocean fish will need to adapt or move to avoid death.

Most prior research has focused on the capacity for animals to adapt to increasing temperatures, given that animals have adapted to changes in temperature in the past. However, previous adaptations happened at evolutionary timescales, roughly one degree Celsius temperature increase per million years. Current predictions for rising temperature are much greater, with sea surface temperature predicted to increase by 2-4 degrees Celsius by the end of the 21st century.

Instead of looking into how fish can adapt, the new research took a different approach by asking, what if fish moved? In fact, what if entire ecosystems were capable of moving to the cooler temperatures, towards the poles or in deeper water?

There is already evidence that many coral reef fish and pelagic fish, like tuna, have moved in response to warmer ocean waters. The researchers found evidence that this also might occur with blue-green damselfish, and they stressed the need to investigate more fish species, including commercial fish species that economies rely upon.

"This study shows, that there is a mechanistic explanation for why fish may move, faced with a choice, and now we have a way of testing it," Johansen said.

Picking up and moving may not be the silver bullet for some species, particularly coral reef fish which are dependent on reefs relocating, too. Blue-green damselfish have a very small range, unable to go more than 60 feet in radius from their coral reef home. Corals cannot move pole-ward as fast as the temperature increases are predicted; if fish do not adapt, reducing critical processes like reproduction, they will have to move to deeper waters where living conditions are less than ideal.

In addition, ocean warming does not occur as a steady slide upward on the thermometer. It often occurs with more severe and frequent heating events. It's these transient warming events that are causing the most damage. This has already been shown in the Great Barrier Reef, when an El Niño caused the temperature to rise above the thermal tolerance of the animals, causing a massive fish kill and widespread coral bleaching.

"It's these transient periods that are causing the most damage," Johansen noted.